If in a vessel with transformer oil two energized electrodes are positioned at a certain distance from each other, at a certain voltage a flashover will occur between the electrodes. The flashover tendency may be minimized by inserting between the electrodes an insulator body which functions as a barrier.
Transformer bushings may comprise an upper insulator and a lower insulator of electric porcelain. At the joint between these there is a flange which is connected to the transformer casing. In the centre of the bushing there is a tube on which is wound a condenser body to obtain a favourable electrical field distribution. The current can be conducted through the tube or a flexible conductor drawn through the tube.
Power transformers which are used in converter plants entail special problems from the point of view of insulation, which somehow have to be overcome in order to ensure a satisfactory function.
In high voltage direct current (HVDC) plants, there is often used at least one converter per pole and station. Normally, also, several bridges are connected in series. One of the poles of a bridge is normally connected to ground and the other pole is connected to the next bridge, thus obtaining a series connection. The direct voltage potential of the respective bridge relative to ground is then increased according to the number of bridges which are connected in series.
Each bridge in the series connection is supplied with alternating voltage from a separate transformer. With increasing direct voltage potential on the bridges relative to ground, the insulation on bushings and windings on the transformers which are connected to the bridges will also be subjected to an increasingly higher direct voltage potential with a superimposed alternating voltage. The insulation of these must therefore be dimensioned so that they are capable of withstanding the increasingly higher insulating stresses to which they are then subjected.
The increasing direct voltage potential leads to special problems which do not exist in transformers used for pure alternating voltage transformation.
For converter transformers, the lower insulator and the transition between the conductor of the transformer winding and the bushing present areas of problems from the point of view of insulation technique. This is described, inter alia, in Power Transmission by Direct Current, by E. Uhlmann, Springer Verlag 1975, pages 327-328.
The electric direct voltage field has a distribution different from that of the alternating voltage field. The distribution of the direct voltage is mainly determined by the resistivity of the various insulating mediums. It is true that both transformer oil, cellulose material and electric porcelain are good insulators, but a certain amount of electric current is conducted in these materials. The relation between the resistivity of cellulose material and transformer oil is about 100. This means that the cellulose in series with oil is subjected to considerably higher fields than the oil, which in turn, therefore, imposes demands for a sufficient amount of solid insulating material in order not to exceed the electric withstand strength. The field distribution as well as the field directions will thus be different from the case with alternating voltage. The current transport also entails a redistribution of charges in the insulating mediums used.
Because of the heavy dependence of the resistivity on moisture content, field strength, temperature, etc., the distribution of direct current is difficult to predict. In addition, the physical nature of the direct voltage, i.e. charge transport, charge, time-dependent behaviour, and so on, gives a picture of the insulation problems arising in connection with HVDC plants, which is very complex and difficult to interpret. In "Space Charge and Field Distribution in Transformers under DC-stress" by U Gafvert and E. Spicar, CIGRE Int. Conference on Large High Voltage Electric Systems, 1986 Session, 12-04, the complexity of the direct voltage distribution is illustrated. As previously mentioned, problems have arisen at the connection between the transformer bushing and the conductor of the transformer winding. This has led to the lower insulator of electric porcelain having to be removed in order to manage the stresses at the HVDC terminal at the higher voltage levels.
No simple explanation of the above phenomenon has been presented. However, there are reasons to suspect that the long surfaces which arise in connection with bushings for high voltages in combination with the direction of the field along the long surfaces are of importance in this connection. Admittedly, also the alternating voltage field is directed along the surface of the lower porcelain, but its physical nature is different. One hypothesis is that the distribution of the direct voltage field runs the risk of becoming unstable and unevenly distributed along sufficiently long surfaces. Another interesting hypothesis is described in an article entitled "Effect of Duct Configuration on Oil Activity at Liquid/Solid Dielectric Interfaces" by R. E. James, F. E. Trick, R. Willoughby in Journal of Electrostatics, 12, 1982, pages 441-447. In this article it is stated that increased charge transport at surfaces caused by turbulence and access to charge is the reason for low electric withstand strength.
As an example of the state of the art there may be mentioned the condenser body in a muff for direct connection of oil cables to transformers, described, inter alia, in Swedish patent specification 214 015 and in ASEA Journal 1963, volume 36, numbers 1-2, page 23. That part of the muff which extends into the transformer is substantially formed as the lower part of a conventional transformer bushing, i.e. with a lower insulator of electric porcelain. The condenser body of the muff is here designed so as to give capacitive voltage control both inwards along the cable end coming from outside and outwards along the porcelain insulator.